1. Field of the Invention
The present invention relates to a plastic impression tray for a lower jaw to be used at a time of taking an impression within an oral cavity of a lower jaw as a preparation stage for preparing a complete denture as a prosthesis in dentistry, wherein the impression tray can prevent a set impression material from floating upward from the tray and being peeled off from the tray, and can be used for taking an impression with high accuracy.
2. Description of the Conventional Art
In a dental care, an impression material such as a silicone impression material or an alginate impression material has been used at a time of taking an impression within an oral cavity as a preparation stage for preparing a prosthesis. For inserting and holding such an impression material within the oral cavity, an impression tray has been used. In the impression taking, the impression material is loaded onto the impression tray so as to be inserted into the oral cavity of a patient, the impression material is pressed into the oral cavity of the patient to be carried out the impression taking, the impression material is set, and the impression material is thereafter taken out from the oral cavity of the patient integrally in a state in which the set impression material is retained in the impression tray.
A large force is necessary at a time of taking out the set impression material from the oral cavity. As a result, there is a problem that the set impression material floats upward from the impression tray and is peeled off from the impression tray, so that an air gap is generated between the impression tray and the impression material. In the case that the air gap is generated, it is often the case that the impression material deforms from the initial set state, and an accuracy of the prepared dental prosthesis is significantly lowered. Further, since the impression material has a property that the impression material is shrinked by a reaction and a volatilization of a water content after being set, the impression material is greatly affected by the shrinkage in the case that the impression material is peeled off from the impression tray, whereby an accuracy of the dental prosthesis is further lowered. Accordingly, for the impression tray, there becomes important a performance that the impression material inserted into the oral cavity of the patient so as to be set can be taken out from the oral cavity of the patient without being deformed.
Generally, the impression tray is provided with a retention hole or groove for mechanically retaining the set impression material. In other words, the impression material paste before being set enters into the retention hole and is set, whereby the set impression material and the tray are mechanically engaged. Particularly, since the set impression material tends to be peeled off in a tray peripheral portion, an application of an undercut (a rib) is carried out along the peripheral portion. However, since a sufficient amount of impression material cannot enter into the undercut portion, the peeling tends to be generated in the peripheral portion. Further, the impression tray is made of a metal or a plastic, and the metal tray can be easily provided with an undercut having a height from 1.5 to 2.5 mm in the peripheral portion by processing as mentioned above. On the other hand, since the plastic tray is affected by a metal mold which is used at a time of producing the tray and the molded tray cannot be taken out, the plastic tray cannot be provided with an undercut portion having an effective height. Therefore, particularly in the impression tray made of a plastic, there has been a problem that the peeling of the impression material from the tray peripheral portion is large, and a deformation becomes larger.
As the tray made of a plastic, there is a dental impression tray which is provided with a bottom wall, and inner and outer walls for forming an impression material retaining concave portion which is similar to a tooth row shape and is formed as a curved shape in a plan view, is provided with a handle portion which extends forward from side ends of front teeth in the bottom wall or the outer wall, and is provided at least in the inner and outer walls of the concave portion with a lot of impression material retention holes which inhibit a relative movement to the impression material over a whole surface (refer, for example, to patent document 1). The patent document 1 exemplifies a structure in which impression material retention holes provided in inner and outer opposed walls are elongated holes which are elongated in a vertical direction and penetrate in an inward and outward direction of the concave portion (refer to claim 2), and the impression material retention hole provided in a front teeth side of the outer wall is provided in the bottom wall of the concave portion or the handle portion, or both of them and is communicated in a penetrating manner (refer to claim 3). However, the patent document 1 shows only the structure in which a distance between the adjacent elongated holes in the drawing is very narrow in the impression material retention holes which are provided in the inner and outer opposed walls and elongated in the vertical direction as mentioned above, and any disclosure of a width of the elongated hole does not exist in the patent document 1. Further, the impression material retention hole provided in the front teeth side of the outer wall is disclosed as the elongated hole in the drawing, however, any disclosure of the distance between the adjacent elongated holes and the width of the elongated hole does not exist. Further, the impression material retention hole having a circular cross sectional shape is provided in the bottom wall, however, a hole diameter and a distance between the adjacent impression material retention holes having the circular cross section shape are not disclosed at all in the impression material retention hole having the circular cross sectional shape. Further, the bottom wall is shown as an aspect that a U-shaped end portion side (a back tooth side) becomes rapidly shallow.
In the dental impression tray described in the patent document 1 mentioned above, it is not assured that the impression material can be sufficiently retained by the impression material retention holes constructed by the elongated holes which are provided in the inner and outer opposed walls and are elongated in the vertical direction, it is not assured that the impression material can be sufficiently retained by the impression material retention hole which is provided in the front teeth side of the outer wall, and it is not assured that the impression material can be sufficiently retained by the impression material retention hole which is provided in the bottom wall and has the circular cross sectional shape. Further, there is a defect that the impression material in the end portion side (the back tooth side) of the U-shaped form tends to be peeled off.